Ignition timing control system for internal combustion engines

ABSTRACT

It is discriminated in an ignition timing control system for an internal combustion engine whether the ignition timing arithmetically calculated on the basis of sensed operating conditions of the engine lies within a predetermined range of ignition timing. When the result of discrimination indicates that the calculated ignition timing lies within the predetermined range, ignition occurs at that instant, while when the result of discrimination indicates that the calculated ignition timing does not lie within the predetermined range, the result of calculation is disregarded, and ignition occurs at a predetermined instant.

BACKGROUND OF THE INVENTION

This invention relates to an ignition timing control system for aninternal combustion engine, which comprises a function for limiting theignition timing to within a proper range so as to ensure occurrence ofignition at an instant lying within the proper range of ignition timing,thereby preventing mis-ignition or ignition at an excessively advancedor retarded instant tending to result when the means for arithmeticallycalculating the ignition timing makes miscalculation or when theignition timing is retarded in a transient condition of engineoperation.

Ignition timing control systems of very complex structure capable ofmore accurate control of ignition timing than hiherto are now demandedto deal with the severe regulations on the engine exhaust gases, andalso, to improve the fuel consumption of internal combustion engines. Inan effort to meet the above demand, researches and studies are nowdirected to electronic ignition timing control systems which provide ahigher freedom of ignition timing control than the conventionalmechanical ignition timing control systems. Among various kinds ofelectronic ignition timing control systems proposed hitherto, anelectronic ignition timing control system of the kind employing amicrocomputer or microprocessor capable of arithmetically calculatingthe ignition timing for the complex control of the ignition timing isproved to be predominant, and reserches and studies are being directedto the production of practically usable model of such an electronicignition timing control system.

However, various problems remain still to be solved before theelectronic ignition timing control system of this kind can be put intopractical use. The reliability is the most important problem among them.That is, although the electronic ignition timing control system of thekind employing the microcomputer or microprocessor can attain thedesired complex control of the ignition timing in compliance with theoperating requirement of the engine, it has such a drawback thatmis-ignition tends to occur in a transient condition of engine operationsuch as an abrupt increase or decrease in the rotational speed of theengine, or ignition tends to occur at an excessively advanced orretarded instant. This is because misreading of input data ormiscalculation of input data leads to an erroneous result of arithmeticcalculation of ignition timing which will provide an inexactly timedoutput signal for the control of the primary winding of the ignitioncoil. Further, various sensors provided for sensing the operatingconditions of the engine, for example, an intake manifold vacuum sensor,an intake air flow sensor and others will not be readily responsive tosuch variables but will respond more or less with a delay time. In suchcases, the operability of the engine will be greatly impaired, thecatalyst will be deteriorated due to the mis-ignition, and the enginewill be forced to stop during running.

SUMMARY OF THE INVENTION

With a view to obviate the problems pointed out above, it is a primaryobject of the present invention to provide a novel and improved ignitiontiming control system of electronic type for an internal combustionengine which comprises, besides the conventional means forarithmetically calculating the ignition timing, means of simpleconstruction for continuously checking the ignition timingarithmetically calculated by the ignition timing calculating means sothat the instant of ignition can be forcedly limited to within theproper range of ignition timing even when ignition is liable to occur atan instant outside the proper range of ignition timing, and also, evenwhen mis-ignition is liable to occur due to a faulty operation of theignition system.

According to the ignition timing control system of the presentinvention, ignition can reliably occur at an instant lying within apredetermined range of ignition timing between a pre-set uppermostinstant of ignition advance (a maximum advance) and a pre-set lowermostinstant of ignition advance (a minimum advance) even when the result ofarithmetic calculation of the ignition timing by the ignition timingcalculating means does not lie within this predetermined range. Theignition timing control system according to the present invention istherefore advantageous in that it can highly reliably prevent anexcessive advance of the ignition timing as well as an excessive retardof the ignition timing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the ignition timingcontrol system according to the present invention.

FIG. 2 is an electrical circuit diagram of part of the control systemshown in FIG. 1.

FIG. 3 shows waveforms appearing at various parts of the control systemshown in FIG. 1 to illustrate the operation of the control system.

FIG. 4 is an electrical circuit diagram of part of a second embodimentof the present invention.

FIG. 5 is a graph showing how the ignition timing is limited to within apredetermined range by the circuit shown in FIG. 4.

FIG. 6 is an electrical circuit diagram of part of a third embodiment ofthe present invention.

FIG. 7 shows waveforms appearing at various parts of the circuit shownin FIG. 6 to illustrate the operation of the circuit.

FIG. 8 is an electrical circuit diagram of part of a fourth embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings showing preferred embodiments thereof.

FIG. 1 is a block diagram of a first embodiment of the ignition timingcontrol system according to the present invention. Referring to FIG. 1,an angle detector 1 is mounted on the distributor shaft in afour-cylinder four-stroke cycle internal combustion engine to generate areference signal T and a crank angle signal CA. The reference signal Tis a train of pulses each of which has a constant pulse width Tθ andwhich appear at a rate of 4 during one complete revolution of thedistributor shaft, as shown in FIG. 3(a). The crank angle signal CA isalso a train of pulses appearing at a rate of 720 during one completerevolution of the distributor shaft, as shown in FIG. 3(b). A pressuresensor 2 senses the negative pressure or vacuum in the intake manifoldof the engine. An engine condition sensor 3 senses one of the operatingconditions of the engine, for example, the temperature of engine coolingwater, the temperature of ambient atmosphere, the pressure of ambientatmosphere or the rate of exhaust gas recirculation (EGR), except therotational speed of the engine and the vacuum in the intake manifold.

An input circuit 4 is connected to the angle detector 1, the pressuresensor 2 and the engine condition sensor 3 to convert the angle signalindicative of the rotational speed of the engine, the analog signalindicative of the intake manifold vacuum and the analog signalindicative of one of the engine conditions into corresponding digitalsignals respectively. An ignition timing computing circuit 5 isconnected to the input circuit 4 to act as a means for arithmeticallycalculating the ignition timing. An output circuit 6 is connected to theignition timing computing circuit 5 and to the angle detector 1 togenerate a train of pulses each of which rises at the arithmeticallycalculated ignition timing and falls at the timing of starting to supplyprimary current to an ignition coil in a known ignition device 8described later. A fail-safe circuit 7 is connected to the outputcircuit 6 and to the angle detector 1 to act as a means for limiting theignition timing to within a predetermined range. This fail-safe circuit7 checks continuously the ignition timing arithmetically calculated bythe ignition timing computing circuit 5 to determine that the result ofarithmetic calculation by the ignition timing computing circuit 5represents the proper ignition timing when the result of arithmeticcalculation lies within the predetermined range of ignition timing, anda corresponding signal is applied to the output circuit 6. When, on theother hand, the result of arithmetic calculation by the ignition timingcomputing circuit 5 does not lie within the predetermined range ofignition timing, the fail-safe circuit 7 selects a predetermined settingas the proper ignition timing and applies a corresponding signal to theoutput circuit 6. In the known ignition device 8 connected to the outputcircuit 6, current is supplied to the primary winding of the ignitioncoil at the fall time of the output signal of the output circuit 6, andthe current flowing through the primary winding of the ignition coil isinterrupted at the rise time of the output signal of the output circuit6, thereby generating an ignition spark jumping across the spark gap ofeach individual spark plug.

The ignition timing computing circuit 5 in FIG. 1 is of a knownconstruction comprising a microprocessor (not shown), and the inputcircuit 4 and the output circuit 6 are connected by bus lines whichtransient control signals applied from the microprocessor. In responseto the application of the data signals indicative of the rotationalspeed of the engine, the intake manifold vacuum and another specificengine condition from the input circuit 4 by way of the bus lines, themicroprocessor in the ignition timing computing circuit 5 arithmeticallycalculates the ignition timing and applies the data signal indicative ofthe arithmetically calculated ignition timing to the output circuit 6 byway of the bus line.

The detailed structure of the output circuit 6 and the fail-safe circuit7 in FIG. 1 will be described with reference to FIG. 2. Referring toFIG. 2, the output circuit 6 comprises a counter 61 receiving thereference signal T and the crank angle signal CA as a reset input and aclock input respectively, a comparator 62, a memory element 63 (whichwill be referred to hereinafter as a latch and which is, for example,Model CD4042 manufactured by the RCA Corporation) connected to theignition timing computing circuit 5 for holding the digital data signalindicative of the result of arithmetic calculation, another counter 64receiving the output signal of the fail-safe circuit 7 and the crankangle signal CA as a reset input and a clock input respectively, anothercomparator 65, a constant setting circuit 66 for setting the angularrange of interruption of current supplied to the ignition coil, and apair of NOR gates 67 and 68 constituting a flip-flop FF1.

The fail-safe circuit 7 comprises a counter 71 receiving the referencesignal T and the crank angle signal CA as a reset input and a clockinput respectively, an AND gate 72 having a plurality of input terminalsfor receiving the output signal portions appearing from a plurality ofoutput terminals Q₀ to Q₆ respectively of the counter 71, a monostablemultivibrator 73 generating a trigger pulse of predetermined pulse widthat the rise time of the output signal of the AND gate 72 to establish anuppermost instant of ignition advance or maximum advance, a pair of NORgates 74 and 75 constituting a flip-flop FF2, another pair of NOR gates76 and 77 constituting another flip-flop FF3, pair of AND gates 78 and79, and an OR gate 700.

The operation of the first embodiment of the ignition timing controlsystem according to the present invention will be described withreference to a time chart shown in FIG. 3. As shown in FIG. 3(a), thereference signal T generated by the angle detector 1 is a train ofpulses each having a predetermined angular interval Tθ, and twoconsecutive pulses appear during one complete revolution of thecrankshaft, with one of the pulses appearing at the top dead center ofthe piston in each individual cylinder or at the crank angle of 0° andthe next appearing at the crank angle of 180°. Also, as shown in FIG.3(b), the crank angle signal CA generated by the angle detector 1 is atrain of pulses each corresponding to 1° of the crank angle.

The pressure sensor 2 is mounted to sense the negative pressure orvacuum in the intake manifold of the engine, and its analog outputvoltage is variable depending on the negative pressure or vacuum in theintake manifold. The engine condition sensor 3 may be a cooling watertemperature sensor of thermistor type or contact type when, for example,the temperature of engine cooling water is sensed for the purpose ofignition timing control. The cooling water temperature sensor 3generates an analog output signal when it is of the thermistor type andan on-off output signal when it is of the contact type.

The output signals from the angle detector 1, pressure sensor 2 andengine condition sensor 3 are applied to the input circuit 4. In theinput circuit 4, the number n of clock pulses of a fixed frequencygenerated during the predetermined angular interval Tθ of rotation ofthe engine is counted to provide a digital signal indicative of thecount n, and the analog signal indicative of the intake manifold vacuumsensed by the pressure sensor 2 is A-D converted to provide acorresponding digital signal. Also, the analog signal indicative of thespecific engine condition sensed by the engine condition sensor 3 is A-Dconverted into a corresponding digital signal. These digital signals areapplied from the input circuit 4 to the microprocessor in the ignitiontiming computing circuit 5 by way of the individual bus lines. It isapparent that such signals are applied to the ignition timing computingcirciut 5 every ignition cycle. Each time these digital signals areapplied from the input circuit 4 to the ignition timing computingcircuit 5, the microprocessor arithmetically calculates the ignitiontiming suitable for the sensed operating conditions of the engine, andan output signal indicative of the result of arithmetic calculation isapplied from the ignition timing computing circuit 5 to the outputcircuit 6 by way of the bus line.

Referring to FIG. 2, the signal indicative of the result of arithmeticcalculation in the ignition timing computing circuit 5 is applied to thelatch 63 which holds the data obtained in each ignition cycle. Thereference signal T shown in FIG. 3(a) and the crank angle signal CAshown in FIG. 3(b) are applied to the counter 61. A pulse of thereference signal T resets or clears the counter 61, and then, thecounter 61 starts to count pulses of the crank angle signal CA, each ofwhich corresponds to 1° of the crank angle. The output signal of thelatch 63 and the output signal of the counter 61 are applied to thecomparator 62. When the output signal of the counter 61 representing thecount coincides with the output signal of the latch 63 representing thecalculated ignition timing data, a pulse as shown in FIG. 3(f) appearsat the output of the comparator 62. The rise time of this pulsecorresponds to the ignition timing arithmetically calculated in theignition timing computing circuit 5, and this signal is applied to thefail-safe circuit 7. The reference signal T and the crank angle signalCA are also applied to the counter 71 in the fail-safe circuit 7. Apulse of the reference signal T resets or clears the counter 71, andthen, the counter 71 starts to count pulses of the crank angle signalCA. The counter 71, the AND gate 72 and the monostable multivibrator 73are provided to determine an uppermost instant of ignition advance ormaximum advance of the ignition timing, and a pulse as shown in FIG.3(c) appears at the output of the monostable multivibrator 73.

A pulse as, for example, shown by the broken waveform or one-dot chainwaveform in FIG. 3(f) may appear from the comparator 62. Such a pulsedoes not lie within the range between a pulse of the output signal ofthe monostable multivibrator 73 establishing the uppermost instant ofignition advance and a pulse of the reference signal T establishing thelowermost instant of ignition advance. It will be seen in FIG. 3(f) thatthe pulse shown by the broken waveform appears outside the above range.In such a case, a pulse as shown by the broken waveform in FIG. 3(d)appears at the output of the flip-flop FF2 composed of the NOR gates 74and 75. It will be seen in FIG. 3(d) that this pulse rises at the risetime of the output signal of the monostable multivibrator 73 and fallsat the rise time of the output signal of the comparator 62. Since such apulse is applied to the AND gate 78 to open the same, this AND gate 78is kept open for that period of time, and the pulse of the referencesignal T appears at the output of the AND gate 78 to be applied to theOR gate 700. In the case of the pulse shown by the one-dot chainwaveform in FIG. 3(f), no change occurs in the output signal of theflip-flop FF2. In the meantime, a pulse as shown in FIG. 3(e) appears atthe output of the flip-flop FF3 composed of the NOR gates 76 and 77. Itwill be seen in FIG. 3(e) that this pulse rises at the rise time of theoutput signal of the monostable multivibrator 73 and falls at the risetime of the reference signal T. Therefore, when the output signal of thecomparator 62, shown by the broken waveform in FIG. 3(f), appearsoutside the range between the output signal of the monostablemultivibrator 73 shown in FIG. 3(c) and the reference signal T shown inFIG. 3(a), no pulse appears at the output of the AND gate 79.Consequently, the pulse of the reference signal T shown by the brokenwaveform in FIG. 3(g) appears at the output of the OR gate 700.

When, on the other hand, the output signal of the comparator 62 appearswithin the range between the output signal of the monostablemultivibrator 73 and the reference signal T, as shown by the solidwaveform in FIG. 3(f), a pulse as shown by the solid waveform in FIG.3(d) appears at the output of the NOR gate 74. Therefore, no pulseappears at the output of the AND gate 78, and the output signal of thecomparator 62 appears directly at the output of the AND gate 79.Consequently, a pulse as shown by the solid waveform in FIG. 3(g)appears at the output of the OR gate 700 in such a case. Such an outputsignal of the OR gate 700 is applied to the output circuit 6 again. Thecounter 64 starts to count pulses of the crank angle signal CA from thefall time of the output signal of the OR gate 700, and when the countattains the numerical value representing the angular range ofinterruption of current supplied to the ignition coil, which range ispre-set in the constant setting circuit 66, a pulse as shown by thesolid waveform in FIG. 3(h) appears at the output of the comparator 64.This output signal of the comparator 65 is applied, together with theaforementioned output signal of the OR gate 700, to the flip-flop FF1composed of the NOR gates 67 and 68, and a pulse as shown by the solidwaveform in FIG. 3(i) appears at the output of the flip-flop FF1. Itwill be seen in FIG. 3(i) that this pulse rises at the rise time of theoutput signal of the OR gate 700 and falls at the rise time of theoutput signal of the comparator 65. This output signal of the flip-flopFF1 is applied to the known ignition device 8. Thus, current starts toflow through the primary winding of the ignition coil at the fall timeof the output signal of the NOR gate 67 i.e., the flip-flop FF1 and isinterrupted at the rise time of that signal, thereby inducing a highvoltage across the secondary winding of the ignition coil for ignitingeach individual cylinder of the engine.

It will be understood from the above description of the first embodimentof the present invention that the signal representing the result ofarithmetic calculation in the ignition timing computing circuit 5 isapplied to the output circuit 6, and the ignition timing pulse signalappearing from the output circuit 6 is applied to the fail-safe circuit7. When the ignition timing pulse appears within the predeterminedrange, that is, in this embodiment, within the range between theuppermost instant of ignition advance or maximum advance determined bythe output signal of the monostable multivibrator 73 and the lowermostinstant of ignition advance or minimum advance determined by thereference signal T, ignition occurs at the instant arithmeticallycalculated by the ignition timing computing circuit 5, that is, at thepoint S₁ shown in FIG. 3(i). When, on the other hand, the ignitiontiming pulse does not appear within this predetermined range, ignitionoccurs at the instant determined by the reference signal T, that is, atthe point S₂ shown in FIG. 3(i), which point indicates the lowermostinstant of ignition advance or minimum advance.

In the first embodiment of the present invention described in detailhereinbefore, the uppermost instant of ignition advance is set at thefixed angle before the top dead center of the piston determined by thecombination of the counter 71, the AND gate 72 and the monostablemultivibrator 73, and the lowermost instant of ignition advance is setat the angle of the top dead center of the piston determined by thereference signal T. Thus, the range of the ignition timing is maintainedconstant independently of the operating conditions of the engine.However, the range of the ignition timing may be made variable dependingon the operating conditions of the engine. In other words, this rangemay be narrowed so that ignition can occur at an instant more suitablefor the operating conditions of the engine when the result of arithmeticcalculation or the calculated instant does not lie within the range ofthe ignition timing.

FIG. 4 shows a second embodiment of the present invention comprisinganother form of the fail-safe circuit 7 preferably used to satisfy theabove requirement. FIG. 5 shows an uppermost instant U and a lowermostinstant L of ignition advance variable depending on the rotational speedof the engine. In FIG. 4, the same reference numerals are used to denotethe same or equivalent parts appearing in FIG. 2. Referring to FIG. 4,the fail-safe circuit 7 comprises memory elements (ROM's) 7a and 7dstoring pre-set or programmed data of the uppermost and lowermostinstants respectively of ignition advance shown in FIG. 5, comparators7b and 7e, and counters 7c and 7f receiving the reference signal T asits reset input and the crank angle signal CA as its clock input. A datasignal indicative of the detected rotational speed of the engine isapplied to the ROM's 7a and 7d from the input circuit 4, and datasignals indicative of the pre-set uppermost instant U and lowermostinstant L of ignition advance corresponding to the detected rotationalspeed of the engine are applied from the ROM's 7a and 7d to thecomparators 7b and 7e respectively. The counters 7c and 7f start tocount pulses of the crank angle signal CA after they are reset orcleared by a pulse of the reference signal T, and output pulse signalsappear from the comparators 7b and 7e when their counts coincide withthe data read out from the ROM's 7a and 7d respectively. The outputsignal of the comparator 7b represents the uppermost instant U shown inFIG. 5, and the output signal of the comparator 7e represents thelowermost instant L shown in FIG. 5. Therefore, the range of theignition timing is given by the hatched zone shown in FIG. 5, andignition occurs at the lowermost instant of ignition advance when theresult of arithmetic calculation in the ignition timing computingcircuit 5, that is, the calculated instant of ignition does not liewithin this zone.

In the aforementioned first embodiment of the present invention, theignition timing and the timing of starting to supply current to theprimary winding of the ignition coil are determined by counting thenumber n of pulses of the crank angle signal CA generated from the angledetector 1. It is, however, also possible that the rotational speed ofthe engine is detected to be converted into clock pulses which arecounted from the position of a reference pulse T so as to determine theignition timing and the timing of starting to supply current to theprimary winding of the ignition coil by utilizing such time (clock)pulses. In such a case too, the uppermost instant and lowermost instantof ignition advance can be set by the fail-safe circuit 7.

Further, although a microprocessor is used in the ignition timingcomputing circuit 5 in the first embodiment of the present invention, awired logic comprising a digital circuit and/or an analog circuit may beused in lieu of the microprocessor.

The aforementioned first and second embodiments of the present inventionare designed so that ignition occurs at the lowermost instant ofignition advance when the result of arithmetic calculation by theignition timing computing circuit 5 does not lie within thepredetermined range of the ignition timing. However, the arrangement maybe such that ignition occurs at the uppermost instant of ignitionadvance when the calculated instant lies on the advance side beyond theuppermost instant of ignition advance, and ignition occurs at thecalculated instant when the calculated instant lies within thepredetermined range of the ignition timing, while ignition occurs at thelowermost instant of ignition advance when the calculated instant lieson the retard side beyond the lowermost instant of ignition advance orwhen no signal indicative of the result of arithmetic calculationappears.

FIG. 6 shows a third embodiment of the present invention comprisinganother form of the fail-safe circuit 7 preferably used to satisfy theabove requirement. The fail-safe circuit 7 shown in FIG. 6 is amodification of that shown in FIG. 2, and the difference from thefail-safe circuit 7 shown in FIG. 2 will only be described herein.

The structure and operation of this modified fail-safe circuit 7 willnow be described with reference to FIGS. 6 and 7. Referring to FIG. 6, apart of the fail-safe circuit 7 shown in FIG. 2 is modified to include acounter 710, AND gates 711, 719, 720, 721 and 722, a monostablemultivibrator 712, a pair of NOR gates 713 and 714 constituting aflip-flop FF2', another pair of NOR gates 715 and 716 constitutinganother flip-flop FF3', NOT gates 717 and 718, and an OR gate 723. Thereference signal T determining the lowermost instant of ignition advanceshown in FIG. 7(b) is applied together with the crank angle signal CA tothe counter 710 which is connected through the AND gate 711 to themonostable multivibrator 712 which generates a pulse signal as shown inFIG. 7(c). It will be seen in FIG. 7(c) that each pulse of this pulsesignal appears on the advance side of the uppermost instant of ignitionadvance determined by the uppermost instant signal, shown in FIG. 3(a),applied from the monostable multivibrator 73 shown in FIG. 2. The pulsesignal shown in FIG. 7(c) is applied to the flip-flop FF2' as its resetinput, and the reference signal or lowermost instant signal T shown inFIG. 7(b) is applied to the flip-flop FF3' as its reset input. The phaseof the output signal of the comparator 62 (FIG. 2) defining the ignitiontiming arithmetically calculated in the ignition timing computingcircuit 5, relative to the uppermost instant signal shown in FIG. 7(a)and the lowermost instant signal (the reference signal) shown in FIG.7(b), is classified into the following three cases I, II and III shownin FIG. 7(d):

(I) The phase is advanced beyond the uppermost instant of ignitionadvance.

(II) The phase lies between the uppermost and lowermost instants ofignition advance.

(III) The phase is retarded beyond the lowermost instant of ignitionadvance.

In the case I shown in FIG. 7(d) in which the phase of the signalindicative of the result of arithmetic calculation is advanced beyondthe uppermost instant of ignition advance, the NOR gate 714 provides anoutput signal as shown by the one-dot chain waveform in FIG. 7(e). Itwill be seen in FIG. 7(e) that such an output signal falls at the risetime of the output signal of the monostable multivibrator 712 and risesat the rise time of the signal I indicative of the result of arithmeticcalculation. Therefore, the output signal of the NOR gate 714 is in its"1" level at the instant of appearance of the pulse of the uppermostinstant signal, and an output signal as shown by the one-dot chainwaveform in FIG. 7(g) appears from the AND gate 719. At this time, nopulse appears from the AND gate 720, since the NOR gate 716 provides anoutput signal as shown in FIG. 7(f) in which it will be seen that suchan output signal falls at the rise time of the lowermost instant signaland rises at the rise time of the uppermost instant signal. The outputsignal of the NOR gate 714 is inverted by the NOT gate 717, and theoutput signal of the NOR gate 716 is inverted by the NOT gate 718. Theoutput signals of these NOT gates 717 and 718 are applied to the ANDgate 721, and the output signal of this AND gate 721 is applied to theAND gate 722 to which the lowermost instant signal is connected.However, no pulse appears from this AND gate 722 since the lowermostinstant signal is not present at this time. Consequently, the AND gate719 provides an output signal as shown by the one-dot chain waveform inFIG. 7(g), that is, the uppermost instant signal, and this signalappears at the output of the OR gate 723.

Similarly, in the case II, no pulses appear from the AND gates 719 and722 as seen in FIGS. 7(g) and 7(i) respectively in which no solidwaveforms are shown. In this case, the output signal of the AND gate720, shown by the solid waveform in FIG. 7(h), that is, the signalindicative of the result of arithmetic calculation in the ignitiontiming computing circuit 5, appears at the output of the OR gate 723.

In the case III in which the phase of the signal indicative of theresult of arithmetic calculation is retarded beyond the lowermostinstant of ignition advance, or when no signal indicative of the resultof arithmetic calculation appears, no pulses appear from the AND gates719 and 720, and the output signal of the AND gate 722, shown by thebroken waveform in FIG. 7(i), that is, the lowermost instant signal,appears at the output of the OR gate 723.

The structure shown in FIG. 6 may also be applied to the fail-safecircuit 7 shown in FIG. 4. In such an application, the output signal ofthe comparator 7b in FIG. 4 may be applied to the NOR gate 715 and tothe AND gate 719 in FIG. 6 as the uppermost instant signal in lieu ofthe output signal of the monostable multivibrator 73 in FIG. 2, and theoutput signal of the comparator 7e in FIG. 4 may be applied to the NORgate 716 and to the AND gate 722 in FIG. 6 as the lowermost instantsignal in lieu of the reference signal T.

In each of the aforementioned embodiments of the present invention, thefail-safe circuit 7 is merely additionally provided as the fail-safemeans for the output circuit 6 applying its output to the ignitiondevice 8. Therefore, the ignition timing can be reliably limited towithin the predetermined range by merely additionally providing thefail-safe circuit 7 without in any way altering the structure of theignition timing computing circuit 5 and output circuit 6.

Although all of the aforementioned embodiments of the present inventionare constructed to limit the ignition timing signal to within thepredetermined range by the action of the fail-safe circuit 7, the effectsimilar to that described hereinabefore can also be obtained by limitingthe data indicative of the result of arithmetic calculation to within apredetermined range.

FIG. 8 shows a fourth embodiment of the present invention constructed tosatisfy the above requirement. This fourth embodiment is actually amodification of the ignition timing control system shown in FIG. 1 inthat the fail-safe circuit 7 is eliminated, and the output circuit 6comprises means for limiting the data indicative of the result ofarithmetic calculation. The output circuit 6 has a structure as shown inFIG. 8 and comprises data comparators 611, 615, 619 and 622, memoryelements (ROM's) 612 and 616, NOT gates 613 and 617, data selectors 614and 618, counters 620 and 623, a constant setting circuit 621, and apair of NOR gates 624 and 625 constituting a flip-flop FF1', all ofwhich are well known in the art.

Referring to FIG. 8, the ROM's 612 and 616 store pre-set or programmeddata corresponding to the rotational speed of the engine, for example,data of a lowermost instant L and an uppermost instant U respectively ofignition advance variable depending on the rotational speed of theengine as shown in FIG. 5. A data signal indicative of the data of thedetected rotational speed of the engine is applied to the ROM's 612 and616 from the input circuit 4, and data signals indicative of the data ofthe pre-set lowermost and uppermost instants L and U of ignition advancecorresponding to the detected rotational speed of the engine are appliedfrom the ROM's 612 and 616 to the comparators 611, 615 and to the dataselectors 614, 618 respectively. A data signal indicative of the data θof the result of arithmetic calculation in the ignition timing computingcircuit 5 is applied to the comparator 611 and to the data selector 614.The comparator 611 compares the data θ of the result of arithmeticcalculation with the data of the lowermost instant L of ignition advancesupplied from the ROM 612, and the larger one of these data is selectedby the data selector 614. This data signal is applied from the dataselector 614 to the comparator 615 and to the data selector 618. Thecomparator 615 compares the data θ or L supplied from the data selector614 with the data of the uppermost instant U of ignition advancesupplied from the ROM 616, and the smaller one of these data is selectedby the data selector 618. Thus, the data signal indicative of the dataof the lowermost instant L of ignition advance appears at the output ofthe data selector 618 when the data θ of the result of arithmeticcalculation is smaller than the data of the lowermost instant L ofignition advance. On the other hand, when the data θ of the result ofarithmetic calculation is larger than the data of the uppermost instantU of ignition advance, the data signal indicative of the data of theuppermost instant U of ignition advance appears at the output of thedata selector 618, while when the data θ of the result of arithmeticcalculation lies between the data of the lowermost and uppermostinstants L and U of ignition advance, the data signal indicative of thedata θ of the result of arithmetic calculation appears at the output ofthe data selector 618. The output signal of the data selector 618 isapplied to the comparator 619. The counter 620 starts to count pulses ofthe crank angle signal CA after it is reset or cleared by a pulse of thereference signal T, and its count is compared in the comparator 619 withthe data output of the data selector 618. When coincidence is reachedbetween the two inputs, a pulse appears at the output of the comparator619 so that ignition should occur at that instant. In response to theappearance of this signal, the combination of the counter 623, thecomparator 622 and the constant setting circuit 621 determines thetiming or instant of starting to supply current to the primary windingof the ignition coil, as shown in FIG. 3(h). The output signals of thecomparators 619 and 622 are applied to the flip-flop FF1' composed ofthe NOR gates 624 and 625, so that this flip-flop FF1' generates anoutput pulse signal having a pulse width as shown in FIG. 3(i). It willthus be understood that the data θ of the result of arithmeticcalculation is compared with the data of the lowermost and uppermostinstants L and U of ignition advance in the output circuit 6, and thisembodiment is also effective in limiting the ignition timing to withinthe predetermined range.

What is claimed is:
 1. An ignition timing control system for internalcombustion engines comprising:means for detecting operating conditionsof an internal combustion engine; means for calculating an instant ofignition spark supplied to said internal combustion engine; means forestablishing an allowable uppermost and lowermost instants of ignitionspark; means for discriminating whether the calculated instant ofignition spark is inside or outside the allowable uppermost andlowermost instants of ignition spark; and means for generating anignition spark at the calculated instant when the discrimination resultindicates that the calculated instant is inside the allowable uppermostand lowermost instants and at a predetermined instant when thediscrimination result indicates that the calculated instant is outsidethe allowable uppermost and lowermost instants, said predeterminedinstant being identical with either one of the allowable uppermost andlowermost instants.
 2. An ignition timing control system according toclaim 1, wherein said detecting means includes means for detectingrotational speed of said internal combustion engine, and wherein saidestablishing means includes means for storing therein the allowableuppermost and lowermost instants in relation to the detected rotationalspeed.